The present invention relates to a battery charger for collectively charging a plurality of secondary cells, and more particularly, a converter circuit for a battery charger used to charge secondary cells for an electric vehicle.
For effectively and rapidly charging secondary cells used as a power supply in an electric vehicle, a non-contact type battery charger has been proposed which makes use of an electromagnetic induction by a high-frequency transformer.
FIG. 3 is a block diagram of a conventional converter circuit designed for use in a one-converter type battery charger comprising a half-bridged DC/DC converter and a boosting PFC.
As shown in FIG. 3, AC power from an AC source 115 is supplied to a full-bridge rectifier circuit 130 formed from four diodes 111 to 114 for conversion into a DC current. A boosting reactor 105 is connected at one end thereof to a positive terminal of the full-bridge rectifier circuit 130, and at the other end thereof to anodes of two diodes 106 and 107. A switching element 101 is connected at a drain thereof to the cathode of the diode 107, and at source thereof in series to drain of a switching element 102 having a source thereof connected to a negative terminal of the full-bridge rectifier circuit 130. Thus, the switching elements 101 and 102 form together a half-bridge circuit.
Also, the diode 106 is connected at the cathode thereof to a common point of connection between the switching elements 101 and 102.
Further, a series circuit formed from two capacitors 117 and 118 is connected in parallel between the drain of the switching element 101 and the source of the switching element 102. A primary coil 121 of a high-frequency transformer 120 is provided having the primary coil 121 thereof connected between a common point of connection between the capacitors 117 and 118 and the common point of connection between the switching elements 101 and 102.
A full-bridge rectifier circuit 140, formed from four diodes 123 to 126, is connected in parallel to either end of a secondary coil 122 of the high-frequency transformer 120. Also, either end of the full-bridge rectifier circuit 140 is connected to a series circuit formed from three cells 131 to 133 and parallel capacitor 127. Thus, high-frequency power generated at the primary side of the high-frequency transformer 120 is full-wave rectified by the full-bridge rectifier circuit 140, and charged into the cells 131 to 133.
A control circuit 135, formed from a variable-frequency oscillator circuit or VF converter, is connected to the gate and source of each of the switching elements 101 and 102 through a drive circuit to provide a control signal under which the switching elements 101 and 102 are controlled to turn on and off alternately.
More particularly, when the switching element 101 is turned off while the switching element 102 is turned on, an output voltage from the full-bridge rectifier circuit 130 is short-circuited through the diode 106 so that energy is stored into the boosting reactor 105.
Then, when the switching element 101 is turned on while the switching element 102 is turned off, the energy stored in the boosting reactor 105 is charged into the capacitors 117 and 118 through the diode 107.
As the switching elements 101 and 102 are turned on and off alternately under the control signal supplied from the control circuit 135, the high-frequency transformer 120 is supplied at the primary coil 121 thereof with a positive- or negative-going high-frequency current through the capacitor 117 or 118.
The gradient of a current flowing through the boosting reactor 105 when the switching element 102 is turned on is proportional to the output voltage from the full-bridge rectifier circuit 130, so the current flows through the boosting reactor 105 in the discontinuous conduction mode (DCM). If the switching element 102 is kept turned on for a fixed period, the envelope and mean value of an input current is proportional to the output voltage from the full-bridge rectifier circuit 130.
Therefore, the power factor can be made 1.0 with no duty ratio control. However, if the frequency is a fixed one, a voltage across the capacitors 117 and 118 will vary very much depending upon a load current. To control the load voltage, the frequency is controlled by the control circuit formed from the variable-frequency oscillator circuit or VF converter.
Referring now to the block diagram in FIG. 4, a conventional one-converter type battery charger converter circuit having full-bridged switching elements will be explained herebelow.
Although the converter circuit having the full-bridged switching elements is somewhat complicated in circuit configuration, it can effectively generate a double primary voltage.
The full-bridge rectifier circuit 130 and the primary and secondary coils 121 and 122 of the high-frequency transformer 120 are quite the same in circuit configuration as those shown in FIG. 3, and so they will not be described any longer below.
The full-bridge rectifier circuit 130 is connected at the positive terminal thereof to one terminal of each of the two reactors 105 and 206 connected in parallel to each other while the other terminal of these reactors is connected to the positive terminal of the capacitor 116 via the diodes 107 and 108. Also, the negative terminal of the capacitor 116 is connected to the negative terminal of the full-bridge rectifier circuit 130.
A first half-bridge circuit formed from the switching elements 101 and 102, and a second half-bridge circuit formed from the switching elements 103 and 104 are connected at either end thereof in parallel to each other. The common point of connection between these first and second half-bridge circuits is connected to the other terminals of the reactors 105 and 206 through diodes 109 and 110.
The gate and source of each of the four switching elements 101 to 104 forming together the full-bridge circuit are connected through drive circuits to a control circuit 136 formed from a variable-frequency oscillator circuit or VF converter and which provides control signals under which a group of the switching elements 101 and 104 and a group of the switching elements 102 and 103 are controlled to turn on and off alternately.
When the switching element 102 is turned on, the output voltage from the full-bridge circuit 130 is short-circuited through the reactor 206 and diode 109 so that energy is stored into the reactor 206.
When the switching element 102 is turned off, the energy stored in the reactor 206 is charged into the capacitor 116 through the diode 108.
When the switching elements 102 and 103 are turned on simultaneously, the charged potential in the capacitor 116 is applied to the primary coil 121 of the high-frequency transformer 120.
Next, when the switching element 104 is turned on, energy is stored in the reactor 105. When the switching element 104 is turned off, the energy stored in the reactor 105 is charged into the capacitor 116 through the diode 107.
When the switching elements 101 and 104 are turned on simultaneously, the charged potential in the capacitor 116 is applied in a reversed polarity to the primary coil 121 of the high-frequency transformer 120. That is, the switching elements 102 and 104 work as a boosting converter while working as an inverter.
Also, since the booster type converter is made to operate in the discontinuous conduction mode (DCM, the envelope and mean value of an input current are generally proportional to an input voltage, the input power factor is high, and harmonic current is so small as to be dealt with using a small filter for the converter.
However, the one-converter type battery charger is complicated in circuit configuration to improve the properties of the converter. For this reason, many diodes are used and the reactors are provided at the DC side, which leads to a low combined efficiency of the battery charging, increased dimensions of the battery charger structure, and thus an increased manufacturing cost.